1. Field of the Invention
The present invention relates to an on-demand inkjet printing device and an on-demand inkjet printing method for printing characters and/or images for use in a printer, a plotter, a facsimile device, a copying machine or the like.
2. Description of the Prior Art
An on-demand inkjet printhead includes nozzles, pressure chambers, an ink feed system, an ink tank and piezoelectric elements, and injects ink drops via the nozzles by transferring pressures generated by the piezoelectric elements to the corresponding pressure chambers so as to print characters and/or images on a print medium such as a sheet of paper.
The inkjet printing is normally a binary value printing, so that a dithering method, an error diffusion method or the like is used for realizing a halftone printing. In the dithering method, however, the number of dots for forming a unit pixel increases for increasing halftone levels. Thus, a resolution is lowered. On the other hand, in the error diffusion method, a striped pattern of dots largely degrades the quality of an image at a portion with a high brightness. It is possible to remove the foregoing drawbacks in the dithering method or the error diffusion method by enhancing the resolution, which, however, increases the cost of the device.
There is another method of realizing a halftone printing, wherein the darkness of a dot is increased by overprinting the same pixel using a binary value printer. In this event, if an injection amount of one ink drop is set equal to that required in the non-overprinting, the total ink amount per pixel becomes excessive to induce deterioration of the image quality, such as blotting of ink on a print medium. For preventing this, the injection amount of one ink drop is reduced upon overprinting while increasing the number of overprinting times. This, however, lowers the printing speed.
There is another method which realizes a halftone printing by changing amounts of ink drops injected via nozzles of an inkjet printhead. Although it is possible to achieve it by changing diameters of the nozzles of the inkjet printhead, highly precise and costly delicate processing is required for producing such an inkjet printhead.
For solving it, there has been proposed a method, wherein a drive waveform applied to each of piezoelectric elements is controlled to control a pressure in a corresponding pressure chamber so as to controllably change an amount of an ink drop injected from a corresponding nozzle.
There are roughly two methods for applying the drive waveform, i.e. voltage, to the piezoelectric element, in one of which the voltage is constantly applied to the piezoelectric element and in the other of which the voltage is applied to the piezoelectric element only upon injection of an ink drop. In the former method, the stress of the piezoelectric element against the voltage is large so that, for example, an insulation property of the piezoelectric element may be degraded. If the applied voltage is lowered for suppressing the stress, the controllability of the ink injection amount is deteriorated. Further, since the high voltage continues to be applied even in a standby state, the power consumption is also a problem.
When an analog switch is used for feeding the drive waveforms to the piezoelectric elements, such a piezoelectric element that is not used for printing over a long time can not keep a standby state since charges are lost due to discharging. Accordingly, when the printing operation is restarted, the drive waveform rises sharply due to recharging so that the ink may be unwantedly injected or the ink drop injection may become unstable.
On the other hand, for increasing the number of halftone levels, it is effective to increase the number of tone or gradation levels per dot while using the dithering method, the error diffusion method or the like. However, for achieving it, it is necessary to increase the number of drive waveforms to be fed to each of the piezoelectric elements. If a circuit structure as shown in FIG. 9 is used therefor, drive waveform feed circuits and switching elements are required as many as the number of the gradation levels of the dot. This complicates the circuit structure and increases the cost thereof.